I suggest two, the first being frozen squid that you can buy in most tackle shops. Cut it up in rectangular pieces about the size of half your finger nail. This is if you are catching your pinfish with hook-and-line. Also, Berkley Gulp Cutbait in the flat patch form (I attached a photo of Berkley Gulp Cutbait). The Gulp is better than the squid, because it stays on the hook better and the same piece can be used over and over. The Pinfish can not pull it off as easy.

If you are using a pinfish trap, lots of anglers like to use chicken, mullet, threadfin, fish carcass, jack mackerel and all kinds of frozen chum, both store bought and homemade. They even sell a leather-like crab bait used just for pinfish traps. They all work. It is important to make sure the bait stays intact for an extended period to keep a consistent flow of pinfish coming into the trap. Some freeze it and just let the natural thawing process slowly release the bait out. Some put ground up fish in a stocking. But the most important thing is not what bait you use, but the location you choose to catch your pinfish.